1. Field of the Invention
The present invention relates to an image processing apparatus, an imaging device, an image processing system, an image processing method and a computer-readable storage medium storing program code for the image processing method, for performing a gain correction process to make uniform the gain of pixels forming an image which is obtained through radiation imaging using an image pickup device composed of a plurality of pixels.
2. Description of the Related Art
In radiation imaging, an object is irradiated with radiation, and the intensity distribution of radiation transmitted through the object is detected.
Specifically, the following method for radiation imaging is widely used.
A combination of a xe2x80x9cfluorescent screenxe2x80x9d (or a xe2x80x9cintensifying screenxe2x80x9d), which emits fluorescence in response to radiation, and a silver film is set up with an object. The fluorescent screen and silver film and the object are then irradiated. The fluorescent screen converts radiation into visible light and a latent image of the object is formed on the sliver film. The silver film having the latent image of the object is then subjected to a chemical process. The silver film then presents a visible image (a radiation image of the object).
A radiation image thus obtained is an analog photograph, and is used for diagnostic imaging and examination.
Computed radiographic apparatuses (hereinafter referred to as xe2x80x9cCRxe2x80x9d apparatuses) using an imaging plate (hereinafter referred to as xe2x80x9cIPxe2x80x9d) with photostimulable phosphor applied thereon are now in use.
The CR apparatus emits photostimulated luminescence when the IP primarily excited by the irradiation of radiation is subjected to a secondary excitation by visible light such as a red laser. The photostimulated luminescence is detected by a photosensor such as a photomultiplier. Image data (radiation image data) thus obtained is used to output a visible image on a photosensitive material or a cathode ray tube.
The above-mentioned CR apparatus, which is a digital imaging apparatus, may be called an indirect digital imaging apparatus because it requires an imaging process of reading in response to a secondary excitation.
The CR apparatus is an xe2x80x9cindirect imaging apparatusxe2x80x9d because a captured image (through radiation imaging) is not instantly presented at the moment the image is taken. This is also the case as with a technique in which a radiation image is taken as an analog photograph.
There have been recently developed apparatuses which capture a digital radiation image using photoelectric conversion means (image pickup devices such as charge-coupled devices) composed of a matrix of pixels, each being a tiny photoelectric converter or switching element.
Radiation imaging apparatuses having a charge-coupled device or two-dimensional amorphous silicon image pickup device with phosphor deposited thereon are disclosed in, for example, U.S. Pat. Nos. 5,418,377, 5,396,072, 5,381,014, 5,132,539, and 4,810,881.
Since these apparatuses instantly display a captured radiation image, they are called a direct digital imaging apparatus.
The advantage of the direct or indirect digital imaging apparatus over the analog photographing technique is due to filmless operation, a large amount of acquired information through image processing and ease of building a database.
The advantage of the direct digital imaging apparatus over the indirect digital imaging apparatus is due to immediacy. Because of its immediacy, a radiation image through radiation imaging is displayed on the spot. In the clinic field typically in need of urgency, immediacy is important.
In the direct digital imaging apparatus using the charge-coupled image pickup device, the gain of each pixel forming the image pickup device is uniform. To produce a uniform output from the image pickup device with respect to the input image, gain correction is required on a pixel by pixel basis.
Image capturing for gain correction is called calibration. A user typically performs calibration on a regular basis.
Specifically, variations in gain in pixels in the image pickup device change with time under the influence of operational conditions. To acquire a satisfactory output image, a proper calibration must be performed in response to operational conditions of the image pickup device at startup.
During calibration, the imaging device irradiates an entire effective imaging area with an object (a subject) removed. The image thus obtained (hereinafter referred to as a xe2x80x9cwhite imagexe2x80x9d or a xe2x80x9cgain imagexe2x80x9d) is stored. Thereafter, actual radiation imaging (clinical imaging) is performed. Specifically, the object is set up and radiation imaging (clinical imaging) is performed. Gain variations of the image thus obtained (a clinical image) are corrected using the prestored white image.
However, if there is any fault in the white image resulting from calibration in the conventional digital imaging apparatus, that fault migrates to all subsequently taken images until a subsequent calibration.
For example, when the irradiation of an entire image field with the radiation of the imaging apparatus is limited through a radiation diaphragm aperture during calibration, a resulting white image is obtained from an actually irradiated area of the entire field. If gain correction is performed on the actually obtained image using that white image, an incomplete gain correction is performed on an area which was not irradiated during calibration.
Besides the irradiation field limitation due to the radiation diaphragm aperture, the white image can become faulty due to the inclusion of a foreign object.
If the white image used in the gain correction is faulty, the fault migrates to subsequent images to be gain-corrected, resulting in poor gain-corrected images.
It is an object of the present invention to provide an image processing apparatus which presents a white image appropriate for use in the correction of an image of an object.
The present invention in one aspect relates to an image processing apparatus and includes a correction unit for performing a correction process on an image of an object, obtained through image capturing, with a second correction image obtained through image capturing, and a determination unit for determining the fitness of the second correction image for the correction process, wherein the determining unit determines the fitness of the second correction image for the correction process based on a result of a comparison of a first correction image obtained through image capturing and the second correction image.
The present invention in another aspect relates to an image processing method and includes a step of performing a correction process on an image of an object, obtained through image capturing, with a second correction image obtained through image capturing, and a step of determining the fitness of the second correction image for the correction process, wherein the determining step determines the fitness of the second correction image for the correction process based on a result of a comparison of a first correction image obtained through image capturing and the second correction image.
The present invention in yet another aspect relates to a processing software program for performing the function of an image processing apparatus performing a correction process on an image of an object, obtained through image capturing, with a second correction image obtained through image capturing. The program includes program code for a step of determining the fitness of the second correction image for the correction process, wherein the determining step determines the fitness of the second correction image for the correction process based on a result of a comparison of a first correction image obtained through image capturing and the second correction image.
The above arrangements present a white image appropriate for use in the correction process of the image of the object.
Further objects, features and advantages of the present invention will be apparent from the following description of the preferred embodiments with reference to the attached drawings.